1. Field of the Invention
The present invention relates to an electrical energy storage system, and more particularly, a high energy, secondary zinc-chlorine battery.
2. Description of the Prior Art
Electrical energy storage systems or secondary batteries of the type described herein are categorized as being of the so-called high energy density (H.E.D.) type since they are capable of supplying upwards of 50 watt hours of electric power per pound of weight. The high energy capacity and compactness of such (H.E.D.) batteries renders them particularly satisfactory for use as principal or auxiliary sources of electrical energy in both mobile and stationary power plant systems. A metal/halogen/hydrate secondary storage battery of the foregoing type is described in detail in U.S. Pat. No. 3,713,888, issued Jan. 30, 1973 entitled "Process for Electrical Energy Using Solid Halogen Hydrate," which is owned by the same assignee as the present invention. The specific teachings of the aforementioned United States patent application are incorporated herein by reference.
Among the advantages of secondary storage batteries of the metal/halogen/hydrate type is the ability of such systems to be refueled or recharged quickly and efficiently whereby they are again restored to full-charged capacity. In a refuelable storage battery as disclosed in the aforementioned patent, the halogen constituent is replenished by withdrawing a proportion of the aqueous electrolyte and substituting therefor a halogen hydrate which, upon decomposition during normal discharge, liberates elemental halogen and water, whereby the concentration of the metal halide in the electrolyte remains substantially constant during the discharge cycle. The halogen hydrate comprises a compact and extremely convenient method of storing and replenishing the halogen component in the storage system. In a rechargeable storage battery system, a reverse electrical current is applied to the electrode area, whereby the halide is oxidized to the elemental halogen which is recovered as a gas and is reconverted in the presence of water in the corresponding halogen hydrate storage area within the system. In either event, the formation of the halogen hydrate is accomplished under controlled temperature conditions, whereby halogen and water are combined to produce a corresponding solid hydrate. This operation can be performed within the system itself or by a central hydrate processing unit disposed remote from the electrical energy storage system.
During the charging of high energy density secondary batteries utilizing chlorine as the halogen constituent and a highly electropositive metal such as zinc as the electrodes and reactants, an aqueous zinc-chloride electrolyte is circulated through the battery cells, plating out the zinc on an electrode base and generating chlorine at the other electrode. The aqueous zinc-chloride electrolyte passing through the cells carries the chlorine produced during the charging operation out of the electrode area and into the chlorine storage system. In a preferred method of holding chlorine ready for use on discharge, the chlorine is converted to chlorine hydrate, as described in U.S. Pat. No. 3,713,888. During the formation of chlorine hydrate by the methods described therein, chlorine and water moisture are brought together at a cold site, under which conditions, at the appropriate temperature and pressure, chlorine hydrate (Cl.sub.2 .6H.sub.2 O) is formed as a solid. The chlorine hydrate may be removed and held under suitable temperature and pressure conditions to maintain it in the solid state, and it may be fed back into the electrolyte stream being returned to the battery in which stream it can release elemental chlorine, preferably as a dissolved or entrained gas, ready for use as a reactant in the battery during electrical discharge.
The present invention is directed to an improved apparatus and method for producing the chlorine hydrate, which is necessary to replenish the chlorine constituent of the electrolyte on discharge of the battery.